Heathkit AC-11
A small black box with one knob labeled Channel Separation, a coil of gray cable, and one job. For about a decade it turned a mono FM tuner into a stereo one.
Overview
The AC-11 is an FM multiplex converter, which the catalogs also call a stereo decoder. Heathkit's own literature called it a Multiplex Converter. ManualsPlus calls it a stereo decoder. eBay sellers call it a tube multiplexer or a channel separator. It's all the same device. It decodes the multiplex signal that an FM station broadcasts alongside its mono audio and splits it back into left and right channels. ManualsPlus describes the job as decoding FM stereo multiplex signals to enable high-fidelity stereo reproduction from compatible broadcasts. That's a bit grander than the box looks.
It is a vacuum tube design, built in the United States, and sold as a kit, which was the whole point of the Heathkit line. The AC-11 was not a tuner and did not pretend to be one. It was an add-on. The catalog copy makes the pitch plainly: with the new Heathkit AC-11 Multiplex Converter, a listener could receive stereo FM programs from any good quality FM tuner that had provision for a multiplex output. That "any good quality FM tuner" clause is the important part. Heathkit was selling this to people who already owned a tuner, possibly a Heathkit, possibly not, and had no intention of replacing it just to get stereo.
The controls are minimal, as period accessories tended to be. There is a knob on the front marked Channel Separation, reported by an owner on Head-Fi. There is a front dial and labeled switches, and labeled input ports with a coiled gray cable, per an eBay listing. That is the whole interface. No tone controls, no meters, no tuning. The tuner does that work and hands the multiplex signal over.
Key Features
The Channel Separation Control
That single front-panel knob is the most distinctive thing about the AC-11, and the most misunderstood. An owner posting on Head-Fi described having one sitting around without being entirely sure what it was for. That's a common reaction to a box whose only control is labeled with a technical term. The control adjusts how aggressively the decoder separates the left and right channels, which in practice is a tradeoff between stereo width and noise. Period multiplex decoders were not digital. Separation was a matter of circuit alignment, and a trim control on the front panel was a practical concession to the variability of the source material and the tuner feeding it.
Tube Multiplex Decoding
The AC-11 is a tube unit. eBay listings describe it as a tube multiplexer, and 1stopretroshop describes the AC-11B variant as a vacuum tube multiplex adapter for converting an FM mono signal into a stereo signal. The circuit theory explains why the box exists at all. As the Radio Boat Anchor Blog puts it, an FM receiver such as the Heathkit AJ-10 recovers the audio and multiplex information from the FM stereo broadcast, and the multiplex information is then used to decode the left and right stereo channels. The tuner does the receiving and the AC-11 does the decoding. In a modern receiver both live on one chassis, but in the early 1960s they were separate purchases.
Inputs, Outputs, and the Gray Cable
An eBay listing notes labeled input ports and a coiled gray cable. That cable carried the multiplex signal from the tuner into the converter. The AC-11 takes that multiplex feed and outputs decoded left and right channels, which then go to the amplifier. It is a two-in, two-out box in essence, and the labeling on the rear panel tells an owner which is which decades later. Service documentation survives in the form of schematics. Manualmachine hosts a "Heathkit AC11 Schematic," the Internet Archive has a channel separator schematic, and elektrotanya lists a "HEATHKIT AC11MPX SCH." That matters, because a kit-built unit with no schematic is a puzzle box.
Historical Context
The AC-11 arrived when FM stereo was new enough that most tuners in the field were mono. Heathkit's Fall & Winter 1961-62 catalog framed the pitch around exactly that: the converter let any good quality FM tuner with a multiplex output receive stereo programs. The company also built a coordinated system around it. According to the Heathkit Virtual Museum, the AC-11 was designed for use with the AJ-30 tuner, and the AJ-30S combination, an AJ-30 tuner plus AC-11 converter, was sold as a function-coordinated system ready to receive FM stereo.
The converter stayed in the catalog for years. It is documented in the 1966 Heathkit catalog (Electronic kit catalog 810/60). It was not confined to Heathkit tuners, either. A YouTube video description documents a Sherwood S2000 II mono tube FM tuner connected via its multiplex out to a Heathkit AC-11, and a Head-Fi owner reports one that "came connected to my HF-81." So it was a utility, not a system-locked accessory.
Collectibility & Value
Survival is decent and prices are not high. Units turn up on eBay as "as is" listings with no tested condition, and no confirmed current market prices were found in the research. Collectors don't fight over this one. It gets thrown in with a tuner purchase or found in a box at a hamfest. Period coverage was modest but positive. High-Fidelity (July 1962, p.46) reviewed the closely related ACW-11 multiplex adapter and found its harmonic distortion relatively low, with channel separation measuring better than 15 dB up to 5,200 cps before falling off at higher frequencies.
Condition reports are encouraging where they exist. One Head-Fi owner describes a unit that lights up, is in great shape, has no rust, and looks as good as the day it was assembled. A seller on vintaelec.com described an example as looking "more then OK and complete." Beyond that, the record is thin. No confirmed common failure modes and no repair procedures were found. Documentation does exist. Assembly and service information is available through ServiceManuals.net, schematics circulate on Manualmachine, the Internet Archive, and elektrotanya, and an AC-11 alignment discussion thread is referenced via an Antique Radio Forums pin.
One caution for buyers: the AC-11 and the AC-11B are different model names, and the sources don't explain the relationship between them. Do not assume the B suffix means a minor revision or that specs carry across. Treat them as separate items until documentation says otherwise.
eBay Listings
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